The present invention relates to a digital transmission system in which a transmission signal is converted according to a CMI (coded mark inversion) coding rule. Particularly, the invention concerns superposing a sub-information signal on a main signal according to violations of a CMI (coded mark inversion) coding rule.
Generally, in the event when binary signals indicative of information are sent out to a transmission path, it is necessary that the signals be converted into transmission path codes which fit a particular transmission medium and system used. An example of such transmission path codes is a CMI code. A CMI code is such that two bits are allocated to one bit of an input signal, i.e., a bit pattern "01" when the input signal is a "0" and a bit pattern "00" or "11" when the input signal is a "1"; the patterns "00" and "11" being transmitted alternately.
Where an alarm signal or like sub-information signal is to be superposed onto a main signal by use of the CMI code, coding is performed in such a manner as to cause a violation of the CMI coding rule to occur in a particular time slot where the superposition is desired. Specifically, to superpose a sub-information signal on a main signal in a position where the input signal is a "0", an output signal "10" is produced while, to superpose the former on the latter in a position where the input signal is a "1", no alternation of the patterns "00" and "11" occurs in that position i.e., the bit pattern corresponding to the immediately preceding input signal "1" is produced. At a receiving station, conversion is effected to turn "01" and "10" of the CMI code to "0" and "00" and "11" to "1", so a main signal may be regenerated with no regard to presence/ absence of sub-information signals.
Usually, a sub-information bit is superposed on a main signal once per n bits of the main signal, and some different kinds of sub-information signals are multiplexed to constitute a frame of sub-information signal. For example, where four kinds of sub-information signals D0, D1, D2 and D3 are multiplexed with four frame bits (F0, F1, F2 and F3) distributed in the frame, F0, D0, D1, D2, D3, F1, D0, D1, D2, D3, F2, D0, D2, D3, F3, D0, Dl, D2 and D3 are superposed sequentially on every n bits of a main signal. In a subsequent point in the system such as a repeater, in response to detection of a violation of the CMI coding rule, the sub-information signals are separated from the main signal and, then, superposes the separated sub-information signals on the main signal together with sub-information signals which the repeater itself generates, the resultant composite signals being sent out to a transmission path. Meanwhile, a receiving station separates the main signal and the sub-information signals from the incoming signals from the repeater, and detects transmission errors based on the violation. If the number of errors is substantial, the receiving station may switch the transmission path currently in use to a standby transmission path. The problem heretofore pointed out with such a system is that errors are often masked to limit the accuracy of error detection because of a potential coincidence of the error and the sub-information bits or frame bits generated in the repeater.